Vacuum distillation apparatus



Nov. 14, 193 9; K. c. D. HICKMAN.

VACUUM DISTILLATION APPARATUS Original Filed Sept. 5, 1936 KennethCDHickman INVENTOR W. W BY 20% NW ATTORNEYS perature of distillation.

Patented Nov. 14, 1939 UNITED STATES PATENT OFFICE 2,180,050 VACUUMDISTILLATION APPARATUS of Delaware Original application September 5,1936, Serial No. 99,632. Divided and this application October 1, 1937,Serial No. 166,856

2 Claims.

This invention relates to improvements in vacuum distillation apparatus,and especially, to that variety of high vacuum evaporation apparatusknown as molecular distillation apparatus.

All organic substances decompose when subjected to sufflciently hightemperatures and the more unstable substances decompose at relativelylow temperatures. If the temperature of distillation at atmosphericpressure is above the temperature of decomposition, it is customary toreduce the pressure and, thus, reduce the tem- There is, however, alimit to the lowering of the temperature of distillation by reducing thepressure, this limit being reached under conditions of moleculardistillation where there are no molecules of extraneous gas to interferewith the natural course of evaporation. Nothing more can be done toimprove distillation from the pressure angle. Thermal decomposition,however, is not a function of temperature only, but of temperaturemultiplied by time and the next improvement in distillation must come byshortening the time of exposure of the distilland. This is done in theordinary continuous molecular still by allowing the distilland to flowin a thin stream by gravity over vertical distilling columns.Unfortunately, gravity is a fixed force and many of the substances dealtwith are viscous. The time of travel under gravitational pull may bemuch greater than desirable.

This invention has for its object the provision of means for making thetime of distillation on a given area of distilling surface shorter thancan be secured by gravitational flow. For this purpose I propose to usemoving surfaces, the travel of which is under control and is greaterthan the flow of oil would be over the same surface if held stationary.Further, I aim at controlling the thickness of the film on the movingsurface so that it is much thinner than would be possible if the oilwere allowed to spread naturally and fall by gravity.

The above considerations concern the fundamentals of high vacuumdistillation. There are often secondary effects which may become ofprime importance. These effects are produced when solids are passedthrough the molecular still and when dirt and solid deposits adhere tothe distilling surface. By the use of rotating distilling members,coupled with knives, brushes, scrapers, and the like, liquids containingsolids may be applied and removed satisfactorily. Materials whichordinarily form hard deposits on the columns are continuously scrapedaway and the distilling member remains bright throughout thedistillation. Distillation with maximum efficiency entails completeabsence of charred or porous material and it also requires'a highlyreflecting surface so that the loss of heat by radiation shall be at aminimum.

My invention will be readily understood from the following descriptionand the accompanying drawing wherein I have illustrated a sectionalelevation of one embodiment involving the principles of my invention.

Referring to the drawing reference number 2 designates a drum closed atone end and rigidly mounted at that end upon shaft 3. The other end ofthe drum is partially closed by plate 4 which is provided with acircular opening 5, concentric with shaft 3. Numeral 6 designates a bodyof low vapor pressure heating fluid, which is heated by a stationaryelectrical heating element 1 to which current is supplied by lead wires8 projecting through opening 5. Numerals 9 and I0 designate collar-likecylinders which surround drum 2 and are substantially concentrictherewith. The cylinders are welded together at the edges to form anannular space H through which a. cooling fluid is circulated byintroduction through conduit l2 and withdrawal through conduit l3. Th scooling unit serves as a surface condenser and is maintainedsubstantially concentric with evaporating drum ,2 by supports I 4 andI5. The top portion of cylinders 9 and I0 is shaped to a point asillustrated at 20 in order to increase the rate of removal of distillatecondensing in that area.

A trough 2| traverses the length of drum 2 and has mounted within it aroll 22 which is rotated by shaft 23 and which is mounted so that thedistance between its surface and that of drum 2 can be adjusted. Liquidto be distilled is introduced into trough 2| through conduit 24. Anadjustable, knife. like scraper blade 25 is mounted in contact with thesurface of drum 2 and serves to remove undistilled residue and solidswhich drop or flow into conduit l6 and are withdrawn from the still. Aplurality of gutters 26, 21 and 28 traverse the inside surface ofcylinder 9 and serve to collect condensate flowing down the condensingwalls and deliver it to conduits 29, 30 and 3| through which it iswithdrawn from the still. A conduit 32 serves to remove the lastfraction distilling after the sur- 7 entry of gas. The interior spaceincluding that inside drum 2 and that between drum 2 and condensingelement 9 is evacuated by pumps (not shown) connected to conduits l! and38 which are mounted in the end plate 39 of the cover 35.

In operating the apparatus the pumps connected to conduits 31 and 20 arestarted and the system evacuated. The heating fluid 0 is brought toelevated temperature by current passed through heater 1 and the drum 2is caused to rotate in thedirection indicated by the arrow. The entiresurface of 2 is thus heated to distillation temperature, as it passesinto contact with the hot fluid at the bottom of the drum. Cooling fluidis introduced into space ll through conduit l3 and is withdrawn throughI2. Material to be distilled is then passed at a predetermined rate intotrough 2| and roll 23 is caused to rotate in the direction indicated.The distance between roll 23 and drum 2, is adjusted so that a bead ofthe liquid to be distilled forms between them. The distilland is thuspicked up and spread in a thin film upon the surface of drum 2 and iscarried around through the still where complete evaporation of a desiredfrac-- tion or fractions takes place. Evaporated molecules leaving theheated surface of 2 are condensed on the internal walls of 9. Thecondensate flows downward into gutters 28, 21 and 28 and is removedthrough conduits 29, 30, 3| and 32. Undistilled material is removed fromthe surface of the drum by scraper 25 and is withdrawn through conduitIS.

The speed of rotation of roll 22 and drum 2 can be changed to give afilm of varying thickness. If roll 22 is the diameter of drum 2 and theyboth rotate at the same rate, the liquid on the surface of roll 22 willbe spread over a ten fold area on the drum 2 and therefore will be asthick. By doubling the speed of drum 2 the film will be 3& as thick andso forth. The distance between the rolls is preferably small, althoughit can be greater, but should in any case be close enough to maintain abead of liquid during rotation. It will readily be seen that the film isthus of much greater thinness and the speed of distillation is greaterthan where the film thickness is determined by gravity as in processeswhere the liquid flows down a vertical distilling column or a distillingdrum dips into a body of the liquid to be distilled.

It is apparent that many widely different variations of the illustratedapparatus can be used without departing from the spirit or scope of myinvention. For instance, instead of applying distilland to the drum bymeans of a roll, it can be applied by a spray or brush or by means of aplain knife edgegenerally known as a doctor blade. Another method ofapplication is to wipe the distilland on the drum by means of a wickwhich dips into a body of distilland and rubs,

against the surface of the drum. Instead of having an individual coverfor each distillation unit, a plurality of units may be housed under asingle cover. This procedure would materially reduce the amount ofpumping required and minimize pumping problems. The condensing surfacecan also be caused to rotate. This would be of advantage when solidcondensate is obtained since the rotating condensing surface could beprovided with a scraper blade. Equivalents such as endless bands can beused instead of a drum or cylinder.

The use of the scraper makes possible the distillation of materialscontaining, or tending to deposit, solids. Solids deposited on the drumsurface are continuously removed and prevented from interfering with theaction of the evaporating surface. This is a decided advantage sinceimpure mixtures containing proteins, mucilaginous materials, etc., canbe distilled without puriflcation and without interfering with thedistillation. If solids removed are large in quantity and will not flowaway in the residual undistilled liquid they can be forced out by ascrew conveyor or similar means.

. The heating fluid employed to heat the revolving drum must have asufllciently low vapor pressure at the temperature at which it is usedthat it will not give all? vapors resulting in destruction of the vacuumor contamination of the distillate and distilland. The selection ofmaterials for this purpose obviously depends upon the degree of vacuumused and the temperature to which they are heated while in the still.For high vacuum distillation such as for instance at pressures belowabout .1 mm. materials such as vegetable and animal oils, non-volatilehydrocarbon fractions, phthalic acid esters, molten tin, low-meltingbismuth alloys, etc., may be used. When high temperatures are employedand difiicultly volatilizable materials such as animal and vegetableoils are distilled, the heating fluid must have a very low vaporpressure. Suitable fluids for this purpose are the undistilled residueof a vegetable or animal oil which has been distilled under molecularconditions, higher boiling petroleum fractions, lower vapor pressurephthalates, and low melting metals and alloys, etc. Heating of the drumcan of course be accomplished in other ways than by a contact liquidheat transfer agent. For instance, a radiant heater may be mountedinside the drum and in close proximity thereto. The heater may heat onlya portion of the drum with each revolution as with the liquid agentillustrated, or the entire drum may be heated at once. The radiantheater can be heated internally with combustible gases or hot vapors,but electricity is preferred.

The principles of my invention are applicable to processes of vacuumdistillation in general, and particularly to high vacuum-short pathdistillation. By high vacuum is meant any pressure below approximately.1 mm. such as, for instance, .01 mm. to .0001 mm. Short pathdistillation is that type of distillation in which the condensing andevaporating surfaces are located in close proximity and an unconstrictedpath for the travel of vapors from one surface to another is provided.When the pressure is low and the distance of separation is less thanabout the mean free path of the molecules of residual gas thedistillation is a variety of short path distillation known as moleculardistillation. Lowest pressures obtainable give best results, but due todifliculties in producing and maintaining such pressures, they are notused commercially. Pressures of less than about .1 mm. such as .01 mm.to .001 mm. are generally used for molecular or pseudo moleculardistillation, i. e., where the conditions closely approach but are notactually molecular. Distances of less than 12 inches between theevaporating and condensing surfaces and preferably less than 6 inches,such as to 3 inches give best results in all of the above types of shortpath distillations. For a more complete disclosure of the principles ofmolecular distillation reference is made to my U. S. Patents 1,925,559and 1,942,858 and to publications by Washburn, Bur. St. Jour. Res." 2(1929) 476-483 and by Burch Pro. Royal Soc." 123A (1929) 271.

Substances which are normally solid can be distilled by applying them tothe rotating surface after melting or by dissolving or dispersing themin a low vapor pressure liquid solvent.

This application is a division of my application #299,632 filed Sept. 5,1936.

What I claim is:

1. High vacuum distillation apparatus which comprises in combinationwithin a closed system a cylindrical, rotatable evaporating surface, anapproximately cylindrical, positively cooled, condensing surface locatedin close proximity thereto and approximately co-axial therewith, meansfor withdrawing liquid from the condensing surface without admixturewith distilland, means for heating to distillation temperature androtating the evaporating surface, means for maintaining a high vacuum inthe space between the evaporating and condensing surfaces, a mechanicalapplicator capable of applying liquid to be distilled onto theevaporating surface in a substantially thinner film than would result ifthe rotating surface were partially immersed in the liquid and a scraperin close relation with the evaporating surface for removing undistilledresidue therefrom.

2. High vacuum distillation apparatus which comprises in combinationwithin a closed system, a cylindrical, rotatable evaporating surface, anapproximately cylindrical, positively cooled, condensing surface locatedin close proximity thereto and approximately co-axial therewith, meansfor withdrawing liquid from the condensing surface Without admixturewith distilland, means for heating to distillation temperature androtating the evaporating surface, means for maintaining a high vacuum inthe space between the evaporating and condensing surfaces, a; rollapplicator in close proximity to the evaporating surface whose axis isapproximately parallel thereto and which is adapted to be rotatedindependently of the evaporating surface, means for supplying liquidwhich is to be distilled to the roll applicator, means for rotating theroll applicator and means for removing undistilled residue from theevaporating surface.

KENNETH C. D. HICKMAN.

